Is there a way to provide a call to a user-defined program after a plug-and-play flash drive has been inserted and detected by Windows?
No, Microsoft finally figured out that autorun programs are a huge security risk, and no longer will start an application when a drive is inserted.
However, an application that's already installed and running can be notified when a drive is inserted. Is that sufficient? Look up WM_DEVICECHANGE and specifically the DBT_DEVICEARRIVAL w/DBT_DEVTYP_VOLUME case.